The invention relates to the manufacture of semiconductor wafers using chemical vapor deposition (CVD) of tungsten silicide and, in particular, to a process for depositing tungsten silicide by the reduction of tungsten hexafluoride (WF.sub.6) by silane gases.
Because of their stability at high temperature, refractory metal silicides, such as tungsten silicide, have received increasing use in the manufacture of semiconductor integrated circuits, e.g. as Schottky barriers, ohmic contacts, or gate metallizations. For the latter application, a low resistivity tungsten silicide layer is formed on a polysilicon layer. Due to the ever decreasing geometry of devices in an integrated circuit, uniformity of the deposition is of great concern. Similarly, deposition rate is of concern since it determines production rates.
Low pressure CVD of tungsten silicide using a mixture of WF6 and monosilane (SiH.sub.4) is described in Solid State Technology April, 1983, pages 183-185 [1]. Monosilane has a greater tendency to nucleate particles in the gas phase than do the chlorosilanes. The accumulation of silicon within the deposition chamber will eventually fragment, generating particles which can contaminate a wafer. Thus, the use of chlorosilanes, such as dichlorosilane (DCS) (SiCl.sub.2 H.sub.2) will lead to a cleaner deposition chamber.
Other compounds dissociate less readily, such as silicon tetrachloride (SiCl.sub.4), but this causes difficulty in starting the deposition cycle. Intermediate compounds, such as dichlorosilane (SiH.sub.2 Cl.sub.2), have intermediate characteristics. The use of dichlorosilane instead of silane is described in U.S. Pat. No. 4,692,343 [2]; in J. Vac. Sci. Technol. B 6 (6), November--December, 1988, pages 1707-1713 [3]; in Surface and Interface Analysis, Vol 14 (1989), pages 13-17 [4]; and in J. Electrochem. Soc., Vol 136, No. 4, April, 1989, pages 1177-1180 [5].
Alternatively, as described in J. Electrochem. Soc., Vol. 134, No. 5, May, 1987, pages 1220-1224 [6], it is known to use disilane (Si.sub.2 H.sub.6) instead of silane for reducing the WF.sub.6. In this article, results from silane and disilane are compared.
In all of these processes, one is confronted with the conflicting requirements of high deposition rate and uniformity of deposition. Improving one while keeping the other constant is desirable, improving both is very difficult.
Further complicating the process, it is difficult to increase the silicon to tungsten ratio without degrading both deposition rate and uniformity. A high silicon to tungsten ratio, i.e. greater than 2:6, prevents degradation of the silicide film in subsequent steps and poor adhesion to the underlying polysilicon film. A silicon to tungsten ratio greater than 2.6:1 is desirable to suppress oxidation of the tungsten in the silicide film and to control consumption of the underlying polysilicon film during formation of the SiO.sub.2 dielectric. The excess silicon in the silicide film also reduces film stress and provides for more desirable etching characteristics. In the prior art, the ratio could be increased only by decreasing the partial pressure of the WF.sub.6 (or increasing the partial pressure of SiH.sub.4 /SiH.sub.2 Cl.sub.2 /Si.sub.2 H.sub.6). This leads to lower deposition rate and worse uniformity.
One technique for modifying the silicon to tungsten ratio is to deposit a thin layer of silicon prior to depositing the silicide, as described in U.S. Pat. No. 4,737,474. While this provides greater adhesion, it does not modify the silicon content of the bulk of the silicide layer.
In view of the foregoing, it is therefore an object of the present invention to provide an improved process for depositing silicide by the chemical reduction of WF.sub.6.
Another object of the present invention is to provide a process for controlling the silicon to tungsten ratio independently of deposition rate and uniformity.
A further object of the present invention is to provide a silicon to tungsten ratio of 2.6:1 or greater.